Hitherto, inkjet recording systems are broadly divided into two groups, a continuous type and an on-demand type. In the continuous type, ink droplets are continuously generated, charge amounts of the ink droplets used for the formation of image are controlled, and the ink droplets are passed through a electrostatic field formed between deflecting electrodes to control the flight passages thereof. In the on-demand type, ink is discharged only when printing is carried out.
The mainstream inks to be used for such inkjet recording systems are inks prepared by dissolving various water-soluble dyes in water or a solvent composed of water and a water-soluble organic solvent and optionally adding various additives thereto (hereinafter referred to as “aqueous dye ink”). However, in the case where printing is practically carried out using the aqueous dye ink, many drawbacks are encountered in that the ink blurs on recording paper depending on the kind of paper, whereby high-quality print can not be obtained, in that a formed recorded image is poor in water resistance and light fastness, in that drying of ink on recording paper is so slow that streaks occur, and in that a recorded image is deteriorated due to color mixing (color turbidity or color unevenness occurred on the interface when dots having different colors are printed adjacent to each other).
For improving the water resistance and light fastness of recorded image that are the problems of aqueous dye ink as described above, there have been made various proposals to apply pigment based ink comprising fine particles of pigment dispersed in an aqueous dispersion medium or a non-aqueous dispersion medium to the inkjet recording system. For example, inks for inkjet printer comprising a pigment dispersed in a solvent mainly composed of water are proposed (refer to Patent Documents 1 to 5 shown below). However, there is a problem in that since the pigment is insoluble in the medium, dispersion stability of the ink is ordinarily poor to likely cause clogging in a nozzle section.
On the other hand, ink comprising a pigment dispersed in a non-polar insulating solvent (hereinafter referred to as “oil based pigment ink”) has advantages in that it is less in blur due to good absorption into paper and in that a recorded image is good in water resistance. For example, oil based pigment ink in which pigment is finely divided with an alcoholamide based dispersant (refer to Patent Document 6 shown below) and oil based pigment ink in which pigment is finely divided with a sorbitan based dispersant (refer to Patent Document 7 shown below) are proposed. However, such inks still have a problem in that the clogging of ink in a nozzle section is liable to occur, because it is not sufficient to uniformly disperse the pigment particles in the state of fine particles in the non-polar insulating solvent and the dispersion stability thereof is inferior. In addition, there is a severe defect in that the ink is poor in scratch resistance because the pigment itself does not have a fixing ability on recording paper.
For resolving these problems, there are proposed resin dissolution type oil based inks using a resin soluble in the non-polar insulating solvent as both a fixing agent and a pigment dispersant. For example, ink containing a terpene phenol based resin as the above-described resin is proposed in Patent Document 8 (JP-A-3-234772) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). However, the ink is still insufficient with respect to the dispersion stability of pigment and is questionable in its reliability. Moreover, since the resin is dissolved in the non-polar solvent, the resin does not remain in an amount sufficient for completely fixing the pigment on recording paper, so that water resistance and scratch resistance are not sufficient.
Further, inks containing an alicyclic saturated hydrocarbon as a resin soluble in the non-polar insulating solvent are proposed in Patent Document 9 (JP-A-5-202329) and Patent Document 10 (JP-A-5-320551). However, the inks are insufficient in the dispersion stability and scratch resistance and when the amount of resin added is increased in order to ensure the scratch resistance, viscosity of the ink increases to cause a problem in that the ink cannot be discharged.
Thus, for obtaining high-level scratch resistance, it is proposed to coat pigment particles with a resin insoluble or semi-soluble in the non-polar insulating solvent. For example, oil based ink comprising a pigment coated with a resin by microencapsulation, etc. is proposed in Patent Document 11 (JP-A-4-25574). However, since it is difficult to uniformly disperse the pigment-included resin particles in the state of fine particle and the dispersion stability thereof is not sufficient, there is a problem in its reliability as ink. In addition, in recent years, high image quality of photographic image is attained by ordinary inkjet printers using the aqueous dye ink. With respect to the pigment ink, for increasing color forming property and transparency, it is required to make pigment fine as far as possible and to keep the dispersion state thereof stably.
In contrast, however, when the pigment is made finer, crushing of primary pigment particles occurs simultaneously with pulverization of the pigment. Further, since cohesive energy simultaneously becomes large due to increase of surface energy, re-coagulation of the pigment particles is apt to occur. As a result, a problem occurs in that storage stability of the finely divided pigment dispersion is impaired. As described above, with respect to the pigment dispersion used in oil based pigment ink for inkjet printer, pulverization at a higher level is demanded. However, high-level techniques are required for dispersing pigment in the state of fine particle, and it is very difficult to increase the dispersion stability thereof. Therefore, development of oil based pigment ink capable of meeting the above-described requirements has been desired.
A black ink for electrostatic inkjet recording device having a volume resistivity of from 107 to 1011 Ω·m and containing carbon black, a phthalocyanine blue pigment and at least one synthetic resin selected from a hydrocarbon resin, an alkyd resin and an acrylic resin is described in Patent Document 12 (JP-A-2001-279139)
It is required for the binder resin for dispersing and coating a coloring agent to ordinarily have characteristics, for example, (1) that it can sufficiently coat the surface of pigment to form a colored admixture, which reveals an appropriate fluidity upon heat, etc., (2) that it can well disperse a coloring agent in a dispersion medium by coating, (3) that it is as transparent as possible, and (4) that it firmly adheres to a recording medium by fixing to exert sufficient scratch resistance.
In view of the characteristics required to the binder resin, for example, the function of being adsorbed onto a coloring agent to well disperse the coloring agent in the dispersion medium and the function of firmly adhering to a recording medium to exert sufficient scratch resistance, it is ideal that the binder resin has as fundamental components a component solvating with the dispersion medium, a component hardly solvating with the dispersion medium and a component having a polar group. However, it is difficult to find out the binder resin that satisfies all of the characteristics described above.
Patent Document 1: JP-A-2-255875
Patent Document 2: JP-A-3-76767
Patent Document 3: JP-A-3-76768
Patent Document 4: JP-A-56-147871
Patent Document 5: JP-A-56-147868
Patent Document 6: JP-A-57-10660
Patent Document 7: JP-A-57-10661
Patent Document 8: JP-A-3-234772
Patent Document 9: JP-A-5-202329
Patent Document 10: JP-A-5-320551
Patent Document 11: JP-A-4-25574
Patent Document 12: JP-A-2001-279139